The present invention relates to a pedicle screw which is inserted into and fixed to a pedicle of a spinal disease patient, and more particularly, to a pedicle screw in which a screw thread of a clamp screw is formed to have dents and protrusions with trapezoid cross-sections so as to form a predetermined gap with a screw thread of a fixing unit having dents and protrusions with rectangular cross-sections when the clamp screw is combined with the fixing unit, and the screw thread of the clamp screw has different pitches to disperse an external impact so as to further firmly combine the clamp screw with the fixing unit.
In general, spinal diseases refer to representative symptoms in which nerves protruding from left and right sides of each of bones are pressed by damaged disks to cause spinal disease patients pains.
A patient having a spine of which part has been damaged is not able to smoothly do personal activities. Even when the damage to the part of the spine is not serious, the damaged part of the spine is pressed by or contacts another adjacent part to cause the patient pains. Therefore, an operation is to be performed with respect to a patient having a spine of which part is broken down or damaged using an artificial aid in order to support the damaged part.
A spinal artificial aid mainly used in this case includes a pedicle screw and a spinal rod so as to assist a damaged spine in restoring to its normal state, wherein the pedicle screw is inserted onto upper and lower parts of the damaged spine so as to operate as a fixing bar and the spinal road is connected to the damaged spine through the pedicle screw so as to operate as a support bar.
Since a patient lives with semi-permanently implanted with an artificial aid, the artificial aid inserted into the body of the patient is not to be deformed or loosened by longtime activities of the patient.
In other words, the pedicle screw is to be firmly combined with the spinal rod in order to maintain the artificial aid in the body of the patient. Various researches and developments have been conducted in order to solve this.
FIG. 1 is a cross-sectional view of a conventional pedicle screw. Referring to FIG. 1, the conventional pedicle screw includes a screw bar 10, a spinal rod 20, and a head 30. The head 30 includes a U-shaped groove which is connected to the screw bar 10 so as to house the spinal rod 20. A fixing screw thread 31 is formed at an inner surface of the U-shaped groove of the head 30 to be tapered, so that dents of the fixing screw thread 31 decrease toward the center of the head 30.
The pedicle screw further includes a clamp screw 40 which is combined with the head 30. Here, a clamp screw thread 41 is formed at an outer surface of the clamp screw 40 to correspond to the fixing screw thread 31 formed at the inner surface of the U-shaped groove of the head 30. Thus, the clamp screw 40 is fixed into the head 30 through the clamp screw thread 41.
However, when the spinal rod 20 continuously receives load from a movement of a patient, the spinal rod 20 pushes the clamp screw 40 and thus separate from the clamp screw 40. In this case, the clamp screw thread 41 of the clamp screw 40 is firmly tightened to the fixing screw thread 31 of the head 30. Thus, the load is concentrated on positions “A,” and thus the clamp screw thread 41 of the clamp screw 40 is bent so as to push the fixing screw thread 31 of the head 30. As a result, the head 30 becomes apart from the clamp screw 40, which loosens the combination between the head 30 and the clamp screw 40.
Also, the fixing screw thread 31 of the head 30 and the clamp screw thread 41 of the clamp screw 40 receive the load through their surfaces onto which the load is applied. Thus, the fixing screw thread 31 and the clamp screw thread 41 absorb the load not disperse the load. As a result, the combination between the fixing screw thread 31 and the clamp screw thread 41 is further easily loosened.